In the naturally iron-fertilized surface waters of the northern Kerguelen Plateau region, the early spring diatom communitycomposition and contribution to plankton carbon biomass were investigated and compared with the high nutrient, lowchlorophyll (HNLC) surrounding waters. The large iron-induced blooms were dominated by small diatom species belongingto the genera Chaetoceros (Hyalochaete) and Thalassiosira, which rapidly responded to the onset of favorable light-conditionsin the meander of the Polar Front. In comparison, the iron-limited HNLC area was typically characterized by autotrophicnanoeukaryote-dominated communities and by larger and more heavily silicified diatom species (e.g. Fragilariopsis spp.).Our results support the hypothesis that diatoms are valuable vectors of carbon export to depth in naturally iron-fertilizedsystems of the Southern Ocean. Furthermore, our results corroborate observations of the exported diatom assemblagefrom a sediment trap deployed in the iron-fertilized area, whereby the dominant Chaetoceros (Hyalochaete) cells were lessefficiently exported than the less abundant, yet heavily silicified, cells of Thalassionema nitzschioides and Fragilariopsiskerguelensis. Our observations emphasize the strong influence of species-specific diatom cell properties combined withtrophic interactions on matter export efficiency, and illustrate the tight link between the specific composition ofphytoplankton communities and the biogeochemical properties characterizing the study area.

Lateral diffusivity coefficients from the dynamics of a SF6 patch in a coastal environment

The dispersion of a patch of the tracer sulfur hexafluoride (SF6) is used to assess the lateral diffusivity in the coastal waters of the western part of the Gulf of Lion (GoL), northwestern Mediterranean Sea, during the Latex10 experiment (September 2010). Immediately after the release, the spreading of the patch is associated with a strong decrease of the SF6 concentrations due to the gas exchange from the ocean to the atmosphere. This has been accurately quantified, evidencing the impact of the strong wind conditions during the first days of this campaign. Few days after the release, as the atmospheric loss of SF6 decreased, lateral diffusivity coefficient at spatial scales of 10 km has been computed using two approaches. First, the evolution of the patch with time was combined with a diffusion-strain model to obtain estimates of the strain rate (γ = 2.5 10- 6 s- 1) and of the lateral diffusivity coefficient (Kh = 23.2 m2s− 1). Second, a steady state model was applied, showing Kh values similar to the previous method after a period of adjustment between 2 and 4.5 days. This implies that after such period, our computation of Kh becomes insensitive to the inclusion of further straining of the patch. Analysis of sea surface temperature satellite imagery shows the presence of a strong front in the study area. The front clearly affected the dynamics within the region and thus the temporal evolution of the patch. Our results are consistent with previous studies in open ocean and demonstrate the success and feasibility of those methods also under small-scale, rapidly-evolving dynamics typical of coastal environments.

In the biogeochemical dynamics of marine ecosystems, silicon is a major element whose role has, for a long time, been underestimated. It is however indispensable to the activity of several biomineralizing marine organisms, some of which play an essential role in the biological pump of oceanic carbon.
This book presents notions indispensable to the knowledge on the silicon biogeochemical cycle in ocean systems, first of all describing the main quantitative analysis techniques and examination of the major organisms involved in the cycle. The author then moves on to study the most up-to-date processes to control the use of silicon and its regeneration in natural conditions, before mentioning the central role played by this original element in the control of all the biogeochemical cycles in the global ocean. The available information finally enables the global biogeochemical budget of silicon in the marine environment to be quantified.

Production regime and associated N cycling in the vicinity of Kerguelen Island, Southern Ocean

Although the Southern Ocean is considered a high-nutrient, low-chlorophyll (HNLC) area, massive and recurrent blooms are observed over and downstream of the Kerguelen Plateau. This mosaic of blooms is triggered by a higher iron supply resulting from the interaction between the Antarctic Circumpolar Current and the local bathymetry. Net primary production, N uptake (NO3- and NH4+), and nitrification rates were measured at eight stations in austral spring 2011 (October–November) during the KEOPS 2 cruise in the Kerguelen Plateau area. Natural iron fertilization stimulated primary production, with mixed layer integrated net primary production and growth rates much higher in the fertilized areas (up to 315 mmol C m -2 d−1 and up to 0.31 d−1 respectively) compared to the HNLC reference site (12 mmol C m−2 d−1 and 0.06 d−1 respectively). Primary production was mainly sustained by nitrate uptake, with f ratios (corresponding to NO3−-uptake / (NO3− - uptake + NH4+ - uptake)) lying at the upper end of the observations for the Southern Ocean (up to 0.9). We report high rates of nitri-fication (up to ∼ 3 µmol N L−1 d −1 , with ∼ 90 % of them < 1 µmol N L −1 d−1) typically occurring below the euphotic zone, as classically observed in the global ocean. The speci-ficity of the studied area is that at most of the stations, the euphotic layer was shallower than the mixed layer, implying that nitrifiers can efficiently compete with phytoplankton for the ammonium produced by remineralization at low-light intensities. Nitrate produced by nitrification in the mixed layer below the euphotic zone is easily supplied to the euphotic zone waters above, and nitrification sustained 70 ± 30 % of the nitrate uptake in the productive area above the Kerguelen Plateau. This complicates estimations of new production as potentially exportable production. We conclude that high productivity in deep mixing system stimulates the N cycle by increasing both assimilation and regeneration.